"Our results lend support to the importance of fish and omega-3 consumption as part of a healthy diet," said senior study author Dr. Dariush Mozaffarian, dean of the Friedman School of Nutrition Science and Policy at Tufts University, in Boston.

"At a time when some but not other trials of fish oil supplementation have shown benefits, there is uncertainty about cardiovascular effects of omega-3s," Mozaffarian said in a university news release.

Fish are the main dietary sources of omega-3 fatty acids. Fatty fish, such as salmon, trout, anchovies, sardines and herring, are the richest source of these nutrients.

Walnuts, flaxseed oil, canola oil and some other seeds and nuts contain the plant-based omega-3 known as alpha-linolenic acid, according to the U.S. Department of Agriculture.

For the study, the researchers analyzed 19 studies from 16 countries that involved nearly 46,000 people. Of these people, nearly 8,000 suffered a first heart attack over time, which resulted in 2,781 deaths.

Plant-based and seafood-based omega-3s were not associated with a lower risk of non-fatal heart attacks. But they were linked with a roughly 10 percent lower risk of fatal heart attacks, although the study can't prove a direct cause-and-effect relationship.

"These new results, including many studies which previously had not reported their findings, provide the most comprehensive picture to date of how omega-3s may influence heart disease," said study leader Liana Del Gobbo, a postdoctoral research fellow at Stanford University School of Medicine. "Across these diverse studies, findings were also consistent by age, sex, race, presence or absence of diabetes, and use of aspirin or cholesterol-lowering medications."

The study was published June 27 in the journal JAMA Internal Medicine.